Is there a mod that gives you access to Mother Ostankya's special units (spider, bears, bats) once you confederate her?

It's not super immersive/seamless, but this mod should do the trick

https://steamcommunity.com/sharedfiles/filedetails/?id=2857502895

I don't know if there's a mod for Mother O, but there's a mod for Wood Elves that gives you access to Drycha's beasties (feral wolves, bears, etc). It actually gives Wood Elves a new building that unlocks it. Depending on how techy you are, maybe you can edit that mod to work for Mother O.

Also, have you tried building an outpost before confederating? You can probably recruit at least some of those critters through the outpost system, maybe? Not sure.

I actually really hate how this works. I'm playing Wood Elves right now, and Drycha doesn't have a confederation quest, unlike other Wood Elf factions, and once you confederate her, you can't hire the Malevolent variants of the tree spirit units. So all you get are what she has in her armies at the time of confederation.